1. Field of the Invention
The present invention relates generally to a vacuum operated speed range shifting mechanism and, more particularly to a mechanism which may be used to shift a torque transfer case of a four-wheel drive system into low and high range speed modes of operation.
2. Related Art
Four-wheel drive systems for vehicles typically include a torque transfer case which includes an input shaft connected to and driven by the output shaft of the vehicle main transmission, a rear output shaft connected to drive the rear wheels of the vehicle, a front output shaft connected to drive the front wheels of the vehicle, and means for drivingly connecting the input shaft to the front and rear output shafts. Conventional torque transfer cases typically include mechanisms for shifting between low and high speed range modes of operation, where the rear output shaft is directly connected to the input shaft during the high speed mode of operation and is connected to a planetary gear reduction set coupled to the input shaft during the low speed mode of operation. Known speed range shifting devices include asymmetric shift rails disposed within the torque transfer case and associated linkages and actuating means disposed externally to the case. Such systems are relatively expensive due to the linkages required and are at least partially exposed to the environment external of the transfer case. Another known shifting mechanism includes a shift sleeve collar mounted to the rear output shaft which is actuated by a linear screw arrangement coaxially disposed about the shift sleeve and driven by an electric motor. While this system eliminates the linkages of the aforementioned system, the included electric motor is relatively expensive and heavy.
In view of the deficiencies associated with the foregoing range speed shifting mechanisms, applicant's invention is in herein presented.